1. Field of the Invention
This invention relates to drive coils designed to supply a powerful magnetic field to supply power and operational commands to a mismatched, spatially remote receiving coil. More particularly, the invention relates to methods and circuits for efficiently driving a resonating transmitting coil by efficient modulation of the carrier, wherein the modulation can be either amplitude modulation or frequency modulation.
2. General Background and State of the Art
Many applications require or would benefit from improved efficiency in L-C tank circuit oscillations. Achieving such efficiency, however, is problematic for a number of reasons. Such problems may be illustratively presented in the context of a particular, exemplary application. Therefore, although there are many applications which would benefit from an efficiently driven oscillator, the description herein will continue with particular reference to a single exemplary application involving BIOnic Neurons (BIONs).
BIONs are implantable micromodular electrical stimulators that can be located internally within a body. Specifically, BION implants may be placed in or near nerves or muscles to be electrically stimulated. BIONs comprise elongated devices with metallic electrodes at each end that deliver electrical current to immediately surrounding biological tissues. The implantable electronic devices are hermetically sealed capsules having the metallic electrodes attached thereto, and containing electronic circuitry therein. BION implants are about 100 times smaller in volume than conventional implantable electronic devices such as cardiac pacemakers and cochlear implants, resulting in significant physical limits on the general principles of power, data transmission and packaging fundamental to operation of BIONs.
The microelectronic circuitry and inductive coils that control the electrical current applied to the electrodes are protected from body fluids by the hermetically sealed capsule and, additionally, can be covered with a biocompatible coating or sheath for further protection of the capsule. The electronic circuitry typically includes an inductive coil, power storage capacitor, and integrated circuit for performing various functions.
Upon command from an external component, the implanted BION emits an electrical stimulation pulse that travels through the body tissues between and around its electrodes, thereby activating, for example, local nerve fibers as required for particular treatments. The BION microstimulator receives power and control signals by inductive coupling to an externally generated RF magnetic field, which is a practical method for recharging a BION's battery and controlling the timing and parameters of stimulations generation by the BION. This is achieved by inductive coupling of magnetic fields generated by extracorporeal antenna and do not require any electrical leads, as discussed in U.S. Pat. Nos. 5,193,539, 5,193,540, 5,324,316, 5,405,367, and 6,051,017, incorporated herein by reference. By selecting the appropriate strength and temporal patterning of stimulation, a desired therapeutic effect can be achieved.
Unfortunately, the small, narrow shape of BIONs has resulted in stringent requirements for wireless power and data transmission and electromechanical assembly. Developing solutions to meet these requirements has been difficult.
For example, the inductive coupling between a primary inductive coil within an extracorporeal antenna utilized to power a BION and a small, secondary inductive coil within the BION itself is difficult to establish and maintain within the stringent requirements of the BION's power, data transmission, and electromechanical assembly. One reason for this is that the coefficient of inductive coupling between a large primary coil and a distant, small secondary coil across an air gap is very low, typically less than 2%. Therefore, the BION must be assembled such that the length and the cross-sectional area of its receiving coil are maximized. However, the very nature of the BION's necessarily small size establishes strict limits on the BION's receiving coil size.
To compensate for the necessarily weak coupling coefficient which thus results, the strength of the primary RF magnetic field, generated by the extracorporeal antenna, for example, must be made high without incurring excessive power dissipation. Specifically, the extracorporeal antenna must be driven to at least 200-400V, and more ideally, to 500V, in order to generate sufficient power to drive the remote, implanted BION. However, selection of an appropriate oscillator to generate sufficient field strength has been problematic. It will be appreciated by those skilled in the art, of course, that the exemplary application discussed herein, involving BIONs, provides one context only, in which efficiently powered oscillators would provide significant improvement, and it will be readily apparent to those skilled in the art that a number of other applications would also be substantially improved by such an oscillator.
As is well understood in the art, power oscillators are classified according to the relationship between the output voltage swing and the input voltage swing. Thus it is primarily the design of the output stage that defines each class. Specifically, classification is based on the amount of time the output devices operate during one complete cycle of signal swing. This is also defined in terms of output bias current, or the amount of current flowing in the output devices with no applied signal.
Conventional A-Class amplifiers are not efficient enough for field use, as they exhibit significant power dissipation. An alternative choice is an E-Class amplifier, or E-Class oscillator. Class E operation involves oscillators designed for rectangular input pulses, not sinusoidal waveforms. The output load is a tuned circuit, with the output voltage resembling a damped single pulse. Advantageously, a Class-E oscillator operates in a switched mode (ON or OFF) which provides a very high collector efficiency that can theoretically approach 100%. In operation, the energy content, or drive level, of the inter-stage signal applied to such single RF transistor, in combination with a temperature-compensated bias circuit, is optimally set so that the single RF transistor is always sufficiently driven ON or OFF with each cycle of the inter-stage signal, but is not overdriven ON or OFF. Although the high field strength and low power dissipation requirements of BION applications, in, general, might be accomplished by using a Class E amplification with a very high Q (>100) tuned circuit, it has been unclear how to effectively utilize a Class E oscillator in a BION application, because of the BION's two other previously mentioned requirements: power efficiency and data transmission.
These requirements are fundamentally in conflict, as power efficiency requires highly resonant operation of the Class E oscillator, while data transmission requires rapid amplitude modulation of the Class E oscillator. With respect to the rapid amplitude modulation in particular, a problematic feature of the Class E oscillator, in BION applications, is that both the position and duration of the drive pulse are critical. For a coil frequency of 2 MHz, any drive pulse over 125 ns causes excessive power dissipation in the switch without significantly increasing the energy in the coil. However, producing various pulse widths requires additional components that increase the cost and size of the coil driver assembly, which is impractical in BION applications.
In addition to these complications, using a Class E oscillator in BION applications causes additional problems. For example, the flexible shape of the BION may easily be deformed while it is worn by the patient. Such deformities will cause fluctuations in the inductance of the external coil, and a Class E oscillator does not inherently accommodate such fluctuations. Moreover, the electromechanical assembly requirements of BIONs make it desirable to accommodate the driver circuitry on the coil itself. This type of construction makes a typical Class E oscillator unsuitable for BION applications. Further, complicated circuitry is required to change pulse width for achieving desired AM modulation, when utilizing a typical Class E oscillator. Changes in pulse width are undesirable because they cause significant degradation of efficiency, something a battery-operated BION has limited capacity to endure.
Of course, it is again emphasized that while the description herein continues in the context of BIONs as an illustrative mechanism, BIONs are an exemplary application only, and a number of other applications, such as radio communication, metal detectors, mine detection, or power and data transmission to many types of remote devices, would also benefit greatly from an oscillator having an efficient driving mechanism greater than that available in standard Class E oscillators.